Nature-inspired Depolymerization of Soda Lignin by Light-induced Free Radical Promoted Cleavage Using Organic Photocatalyst

The development of sustainable valorization methods for lignin is a challenging task, because the vast majority of the reported methods require either harsh conditions or the use of expensive transition metal catalysts. Inspired from the natural-sunlight degradation of lignin, known as lignin yellowing, we report here the use of a commercially available cheap organic photocatalyst, namely, phenacyl bromide (PAB), for the efficient cleavage of lignin model compound 2-phenoxyacetophenone (2-PAP), and the depolymerization of soda pulped lignin (SL) under UV-A irradiation under ambient conditions. Real-time NMR investigations of the photoreaction between the model compound and PAB shed light on the possible reaction mechanisms involving different radical species, HBr, and molecular oxygen. Interestingly, combined spectral, chromatographic, powder X-ray diffraction and thermal studies of the photocatalytic reaction between PAB and SL indicated the formation of guaiacyl alcohol. The unprecedented catalytic performance of such a simple organic photocatalyst is attributed to the generation of phenacyl radicals, photolabile brominated species and HBr playing key roles in the cleavage of β-O-4 linkages. This study represents a new edge for lignin valorization under mild reaction conditions and offers the opportunity for large-scale environment-neutral production of valuable aromatics using technical lignins as feedstock.